For months, top leaders from business and labor groups wrestled over how to create an immigration program that would allow lesser-skilled workers to come into the country legally.

The fruits of their negotiation became part of an immigration reform bill introduced in the Senate. A new "W visa" program included in the legislation would bring in anywhere from 20,000 to 200,000 workers each year.

Here's why the program is important: for more than two decades, we've tried to stop illegal immigration using bigger fences and workplace raids. But employers keep hiring people, and workers keep coming.

Another option is to create a way for lesser-skilled workers to come here legally. Unions have traditionally opposed guest-worker programs, though, saying that they make workers vulnerable to exploitation and undermine the wages of U.S. workers.

So as a fix, they created a program for lesser-skilled workers that wasn't a guest-worker program. It's one that would potentially allow immigrant workers to eventually transition to become legal permanent residents.

Looking at the immigration reform bill proposed by a group in the Senate, however, it's unclear how realistic of a chance W-visa holders would have at getting permanent legal status.

For starters, they're banned from applying for legal status through a program meant for people who have lived in the U.S. legally for 10 years or longer. So a decade of work would be no guarantee for permanent legal status.

The main route that W-visa holders are expected to use is through the new "merit-based" visa program that ranks immigrant applicants based on a points system.

A portion of merit-based visas appear targeted toward immigrants with lower levels of education, which would likely fit many of those using the W visa. When the program would effectively begin in five years, there would be 60,000 visas available, with the possibility of increasing to 125,000 depending on economic factors.

But it's unclear how much competition there would be for those visas, and whether future waves of lesser-skilled workers would have a good shot at getting them.

Why isn't it clear? That's because the merit-based visas, as proposed, would be awarded based on a complex point system, where applicants will get points for hard-to-quantify things like "civic involvement" and an "exceptional employment record."

Right now, economists don't have the data available that you would need to see how all of that will play out.

There are lots of variables, and even with the data, it could take months to develop an economic model that would project what the new merit-based visa program would do to the U.S. workforce, according to Michael Clemens, a senior fellow at the Center for Global Development, a non-partisan think tank.

The group of senators working on immigration reform haven't released economic reports that show why the new merit-based program is any better than the system we have now, Clemens said.

"Either hard numeric forecasts are not being made, or they're being made but they're not up for public debate because they're not being released," Clemens said.

Without that data, it's impossible to know how other parts of the bill will function, including whether future waves of lesser-skilled workers will have a chance at become permanent residents.

"That's a real problem," Clemens said. "We're talking about massive effects on the U.S. economy."